This invention is related to mirrors, and more specifically, to mirrors used in laser applications.
In the following drawings, similar reference numbers are used to show similar features. FIG. 1 shows a simplified view of a prior art ring laser gyro (rlg) 10. The rlg includes a block 15, bores 30A-C located within the block 15 output mirror 20, transducer mirrors 25 A and B, and electrodes 35 A-C. Located within the bores, but not shown, is a gain medium such as a combination of helium and neon gasses. When the gas is electrically charged via electrodes 35A-C, two primary laser signals are formed. One propagated in a clockwise direction around the path formed by the bores, the other propagated in the opposite direction. In the case of helium neon gas, a laser signal operating at a wavelength of 633 nm is the preferred mode of operation.
The output mirror 20 has a lower reflectivity than the transducer mirrors 25A and B to allow some light to pass through the mirror. Readout optics and electronics can be arranged in communication with the output mirror 20 in a well known way to produce a signal representative of the rotation rate of the rlg.
The transducer mirrors 25A and B have high reflectivitities, on the order of 99.996%, to ensure propagation of the laser signals. The transducer mirrors are so named since they sit on transducers for adjusting the path length of the laser signals. This so called path length control is well known in the art.
High-reflectance mirrors, such as those used in ring laser gyros, are structures consisting of a series of periodically varying refractive index layers in a direction perpendicular to the mirror surface. Examples of such structures are shown in prior art FIGS. 2-5.
In FIGS. 2 and 3 a prior art output mirror 20 is shown. The mirror 20 is constructed of a stack of alternating layers of high and low index of refraction materials, each layer having an optical thickness equal to a quarter wavelength (.lambda./4) of the primary wavelength of light (mode) of the laser. Optical thickness is calculated by determining the path length through the layer along the path by which the laser signal propagates through the layer times the index of refraction. Layers 305A-305M may be a high index of refraction material such as titanium dioxide (TiO.sub.2). Layers 310A-310M may be a low index of refraction material such as silicon dioxide (SiO.sub.2). Substrate 330 is formed from a material having a desired coefficient of thermal expansion such as Zerodur or BK-7, materials which are well known in the art. Apparatus and methods of producing such mirrors are disclosed in U.S. Pat. Nos. 5,216,330 issued on Jun 1, 1993 to Ahonen, 5,240,583 issued on Aug. 31, 1993 to Ahonen and 5,308,461 issued on May 3, 1994 to Ahonen, all of which are commonly owned with the present application and are incorporated herein by reference.
Referring now to prior an FIGS. 4 and 5 there shown is a prior art transducer mirror 25. The transducer mirror may be identical to the output mirror 20 in all respects including thickness, except that the transducer mirrors included more layers to accomplish the desired higher reflectivity.
While the rlg operates on a selected primary mode, additional modes of operation may also be present, such as signals operating at a wavelength of 650 nm, which is P-polarized These additional signals add noise to the signal produced by rotation of an operating rlg. Thus, there have been attempts to eliminate the 650 nm signal. U.S. Pat. No. 4,627,732 (Braun et al.) issued on Dec. 9, 1986 and 4,519,708 (Perimutter et al.) issued May 28, 1985 are both directed toward attempted solutions to the suppression of unwanted modes of laser operation.
In Braun et al., a mirror is treated with an electron beam which induces a change in the physical structure of the mirror. This in turn causes a phase change to occur to undesired signals thereby destroying propagation of the unwanted signal.
In Perimutter et al., a rlg mirror is modified by deposition of a light absorptive material on the surface of the mirror. The absorptive material absorbs some of the energy of the undesired signal thereby preventing further propagation.